1. Field of the Invention
The invention relates to the field of electrically tunable optical or light filters and values in which a filter element is caused to change or switch color or is caused to switch between light transmissive and opaque in response to a chemically transmitted electrical signal.
2. Description of the Prior Art
Optical filters for light in the visible range ordinarily contain colored compounds or thin layers of materials that achieve wavelength specificity through interference effects. These conventional filters have fixed absorption characteristics which cannot be varied, or "tuned," once the filter is constructed.
Rare-earth diphthalocyanines are known from prior publications to have electrochromic properties in which the color of the diphthalocyanine can change over a period of about eight seconds upon application of a potential difference across an electrochemical cell having a diphthalocyanine film on one of the electrodes. P. N. Moskalev and I. S. Kirin, "Effect of the Electrode Potential on the Absorption Spectrum of a Rare-Earth Diphthalocyanine Layer," Opt. i Spektrosk, 29, 414 (1970) and P. N. Moskalev and I. S. Kirin, "The Electrochromism of Lanthanide Diphthalocyanines," Russian J. Phys. Chem., 46, 1019 (1972).
U.S. Pat. No. 4,184,751 of M. M. Nicholson, the inventor herein, describes the use of metal diphthalocyanine complexes as the electrochromically active material in an electrochromic display cell. Rapid color changes in less than 50 milliseconds are achieved, thus alleviating the slow switching time previously reported for rare-earth diphthalocyanine complexes. Power requirements are small because of the low power switching characteristics of the material and because the device exhibits an open circuit memory of from several minutes to several hours, depending on its construction. A multi-color, i.e., more than one color, device is achieved through use of a range of voltages applied between generator and counter electrodes. Color reversal of displayed information and the background against which it is displayed is achieved through use of display electrodes in the background portions of the viewing area as well as in the character segments.